Sagittal rotation determination

ABSTRACT

The invention relates to a method of determining the sagittal rotation of a patient&#39;s pelvis based on a standard anterior posterior X-ray-image with known image parameters and a calibration of the image, for example by using at least one King-Mark calibration object. The angle of the pelvic rotation is determined between a pelvic plane which is orthogonal to the midsagittal plane of the pelvis, and the image plane of the X-ray-image. Assuming the patient&#39;s position shown on the X-ray-image represents a standard neutral position, the X-ray-image plane can be used as a functional reference plane for further calculations, for example during hip-replacement surgery. The present invention further relates to a corresponding computer program and system.

RELATED APPLICATION DATA

This application is a national phase application of InternationalApplication No. PCT/EP2018/055067 filed Mar. 1, 2018, claiming priorityfrom PCT/EP2017/055583 filed Mar. 9, 2017 and published in the Englishlanguage.

TECHNICAL FIELD

The present invention relates to a computer implemented method fordetermining the sagittal rotation of a patient's pelvis and to acorresponding computer program and system.

SUMMARY

In medical procedures concerning the pelvis of a patient, for examplehip-replacement surgery, it is desirable to know how the pelvis isorientated with respect to other anatomical structures such as thefemurs for a specific patient and for different postures, particularlyfor a standing posture of the patient. In the particular case ofhip-replacement surgery it is an ultimate goal to orient the artificialhip-joint components with respect to the pelvis such that the patientcan move in a usual manner after surgery without the hip joints range ofmotion being negatively constrained with respect to the pre-operativerange of motion.

A prior art approach for determining an implant orientation is todetermine the pelvic tilt angle, i.e. the angle between the anteriorpelvic plane (APP) and the patient's coronal plane in a standing postureof the patient, whereupon the anterior pelvic plane is used as areference plane for aligning the artificial joint components. Fordetermining the pelvic tilt, an X-ray-image taken in ananterior-posterior direction can however provide only a very roughestimation of the tilt angle. For determining the pelvic tilt moreprecisely, it is therefore necessary to acquire additional pre-operativeX-ray-images of the standing patient's pelvis in a lateral direction, oreven performing a pre-operative CT-scan. These approaches however inducean increased and therefore unwanted radiation exposure of the patient.

The present invention allows for precisely determining the sagittalrotation of a patient's pelvis without imposing additional radiationexposure on the patient.

The method, the program and the system are defined by the appendedindependent claims. Advantages, advantageous features, advantageousembodiments and advantageous aspects of the present invention aredisclosed in the following and contained in the subject-matter of thedependent claims. Different advantageous features can be combined inaccordance with the invention wherever technically expedient andfeasible. Specifically, a feature of one embodiment which has the sameor a similar function to another feature of another embodiment can beexchanged with said other feature, and a feature of one embodiment whichadds an additional function to another embodiment can in particular beadded to said other embodiment.

According to a first aspect of the present invention a computerimplemented method for determining the sagittal rotation of a patient'spelvis is provided, wherein the method comprises the steps of:

-   -   acquiring image data describing a two-dimensional X-ray-image of        the patient's pelvis made in an anterior-posterior direction;    -   determining, based on the image data, position data describing        the position of a plurality of pelvic landmarks reproduced in or        derivable from the two-dimensional X-ray-image;    -   acquiring calibration data describing the position of a        calibration plane with respect to the patient's pelvis, the        calibration plane being perpendicular to the anterior-posterior        direction;    -   determining, based on the position data and the calibration        data, calibrated projection data describing a two-dimensional        projection representing the plurality of pelvic landmarks of the        actual patient's pelvis being projected into the calibration        plane in the anterior-posterior direction;    -   acquiring distance data describing at least one of a        medial-lateral distance and a cranial-caudal distance between at        least one first pelvic landmark and at least one second pelvic        landmark in the calibrated two-dimensional projection;    -   acquiring, from a database, regression data describing a        correspondence, particularly a linear correspondence between at        least one of said medial-lateral distances and cranial-caudal        distances, and a pelvic sagittal rotation;    -   determining, based on the distance data and the regression data,        the sagittal rotation of the patient's pelvis.

In other words, an X-ray-image of the patient's pelvis is acquired in ananterior-posterior direction, and at least two landmarks are identifiedin the X-ray-image. For example, such landmarks are prominent featuresof the patient's pelvis. Additionally or alternatively, landmarks canalso be calculated from such prominent features; for example, the centerof rotation of the hip joint can be calculated from the location ofpoints on the surface of the acetabulum, which can be identified in theX-ray-image.

Further, a calibration plane is defined with respect to the pelvis, suchthat the landmarks can be projected into that calibration plane. In apreferred embodiment, the calibration plane's position is defined suchthat it includes the centers of rotation of at least one acetabulum.Assuming that the radiation source for acquiring the X-ray-images isrepresented by a punctiform source, it becomes apparent that the imageof the pelvis acquired via such source is somehow “distorted”, dependingon the distance between the radiation source and the depicted featuresof the pelvis. To compensate for such image distortion, landmarks whichare of interest for later procedures are virtually projected into thecalibration plane as if the landmarks of the (actual) pelvis areprojected into the calibration plane solely in an anterior-posteriordirection. In other words, a “parallel projection” of the“landmark-array” of the three-dimensional pelvis into the calibrationplane in an anterior-posterior direction is determined.

With the quasi parallel projection being determined, the projected imageserves for distance measurements within the calibration plane andbetween the at least two landmarks.

Further, a database is provided which comprises a plurality of referencepatient datasets which not only include the same distance measurementsfor a plurality of reference patients, but also a determined value forthe sagittal rotation of the pelvis of each reference patient. Thedatabase therefore provides a correlation betweeninter-landmark-distances within the calibration plane and the sagittalrotation of the pelvis for each reference patient.

A comparison between the distances measured for the current patient andcorresponding distances determined for the reference patients ultimatelyallows for deriving the sagittal rotation angle for the current patientfrom the statistical data provided by the reference database.

According to a first alternative embodiment of the present invention,“meaningful” distance measurements between pelvic landmarks within thecalibration plane are only possible when the exact distance between thecalibration plane and the X-ray-source is determined. For determiningthe position of the calibration plane with respect to the radiationsource, at least one calibration feature of a predetermined size and/orgeometry may be used, which is reproduced in the X-ray-image. The knowndistance between the radiation source and the image plane and the knownsize and/or geometry of such calibration features allows to calculate,from the features' depiction in the X-ray-image, the position of thesefeatures with respect to the radiation source in three-dimensional spaceand therefore defines the position of the calibration plane inthree-dimensional space. It is however also possible to provide suchfeatures in an anterior and in a posterior location with respect to thepelvis. Such calibration features are for example described in U.S. Pat.No. 8,075,184. The calibration features disposed anterior to the pelvismay define a first, anterior reference plane, whereas the calibrationfeatures disposed posterior to the pelvis may define a second, posteriorreference plane. Both positions of the reference planes are known inthree-dimensional space. Knowing the ratio of the A-P-distance from thecalibration plane to the anterior plane and the A-P-distance betweenboth reference planes, the relative position of the calibration planebetween both reference planes can be defined and as result thecalibration plane is defined in three-dimensional space. The ratio ofthe reference planes and the calibration plane are taken from adatabase.

It is however also necessary to know the distance of each of theanatomical landmarks in an anterior-posterior direction with respect tothe calibration plane. The distance of each landmark with respect to thecalibration plane may be taken from a database containing predefinedvalues for the anterior-posterior-distances of a plurality of pelviclandmarks. In the alternative, the A-P-distance can be acquired bymatching a three-dimensional pelvis model which may in turn be acquiredfrom an anatomical atlas, to the two-dimensional X-ray-image. Thispelvis model specifies the position of the plurality of pelvic landmarksand therefore may also specify the A-P-distances of each of the pelviclandmarks with respect to a predefined plane. Therefore, the necessaryA-P-distances of the landmarks of the current patient can be identifiedfrom statistical values provided by that database.

For the above described embodiment, the setup geometry between theX-ray-source, the pelvis landmarks and the image plane is known. Fromthis known setup geometry, “parallel projection” of the pelvis into anyarbitrary calibration plane that is parallel to the image plane can becalculated. This “parallel projection” is now free from any shift ordistortion caused by the “conical” X-ray-projection and finally allowsfor meaningful distance measurements between pelvic landmarks within thecalibration plane.

While the above-described first embodiment requires the distance betweenthe X-ray-source and the detector plane to be determined, the followingalternative second embodiment does not. As already described furtherabove, the calibration features disposed anterior to the pelvis maydefine a first, anterior reference plane, whereas the calibrationfeatures disposed posterior to the pelvis may define a second, posteriorreference plane. For both reference planes the magnificationfactor/distortion factor is known because of the known size of thecalibration feature. Likewise each landmark defines a landmark planewhich is positioned between the two reference planes.

Knowing the ratio of the A-P-distance from one of the calibrationfeatures to one of the landmarks and the A-P-distance between bothcalibration features the relative position of each landmark planebetween the reference planes can be defined. The ratios of the landmarksand calibration spheres are taken from a database.

Due to the projective nature of the X-ray the magnification factorincreases linear between the punctiform source and the detector planeand therefore it also increases linear between the anterior and theposterior reference plane. Because the relative position of the eachlandmark plane between the anterior and posterior reference planes isknown and the relative position correlates linear with the magnificationfactor, the magnification factor of each landmark plane can be defined.

Generally spoken trough each landmark a plane can be drawn which isparallel to one of the reference planes and for each of those planes itis known how an object in this plane is magnified. Therefore the truesize of an object being positioned in one of those planes is known whichallows for meaningful distance measurements between pelvic landmarks.

The landmarks can either be identified manually, for example by apractitioner palpating depictions of the landmarks within the originalX-ray-image or the already “calibrated” image. In the alternative, thelandmarks can be identified automatically by matching athree-dimensional pelvis model acquired from an anatomical atlas to thetwo-dimensional image.

The position of the landmarks within the image can then be derived fromthe anatomical atlas which indicates the position of the landmarkswithin the model of the pelvis.

According to a preferred embodiment of the present invention, the pelviclandmarks are selected from the group consisting of:

-   -   left anterior superior iliac spine, right anterior superior        iliac spine;    -   left iliosacral joint, right left iliosacral;    -   left lateral foramen point, right lateral foramen point;    -   cranial edge of pubic symphysis; and    -   center of rotation of left acetabulum, center of rotation of        right acetabulum.

Of course, any other pelvic landmark that is suitable for distancemeasurements may be acquired, as well.

According to a further preferred embodiment of the present invention,distances between the anatomical landmarks are measured in acranial-caudal/vertical direction and/or in a medial-lateral/horizontaldirection. The distance measurements may be selected from the groupconsisting of:

-   -   medial-lateral distance between left anterior superior iliac        spine and right anterior superior iliac spine;    -   cranial-caudal distance between iliosacral joint and line        connecting anterior superior iliac spines;    -   cranial-caudal distance between center of rotation of left or        right acetabulum and line connecting anterior superior iliac        spines;    -   cranial-caudal distance between center of rotation of left or        right acetabulum and line connecting left lateral foramen point        and right lateral foramen point;    -   medial-lateral distance between center of rotation of left or        right acetabulum and midsagittal plane of patient; and    -   cranial-caudal distance between center of rotation of left or        right acetabulum and cranial edge of pubic symphysis.

Again, any other suitable distance measurement between identifiedlandmarks may be performed in the context of the present invention.

As already set out further above, it is desirable to know the sagittalrotation of the patient's pelvis when the patient takes a standingposture. Even though the present invention can be performed for anyposture of the patient, it is the standing posture which is mostdesirable.

With the necessary distance measurements acquired for the currentpatient, the determined values for each measurement can be compared to astatistical distribution obtained from corresponding measurementsperformed on a plurality of reference patients. From the measurementstaken in a medial-lateral direction, it is possible to derive a commonbasis for comparing a plurality of pelvic bones, irrespective of theactual size of the bone and the measured cranial-caudal distancesresulting therefrom. The measurements taken in a cranial-caudaldirection then give a direct indication how much the pelvis is rotatedin the midsagittal plane. As, according to the present invention, thedatabase further provides a correlation between the measured distancesin the cranial-caudal direction, and the sagittal rotation of thepelvis, the sagittal rotation of the current patient's pelvis can befinally derived from the database.

In particular, the pelvic sagittal rotation may define the angle betweenthe coronal plane of the patient and an AAC-plane containing the leftanterior superior iliac spine, the right anterior superior iliac spine,and at least one of the center of rotation of the left acetabulum andthe center of rotation of the right acetabulum.

Further, the determined AAC-plane may serve as a reference basis for aregistration procedure for surgery, particularly for hip replacementsurgery.

Definitions

The method in accordance with the invention is for example a computerimplemented method. For example, all the steps or merely some of thesteps (i.e. less than the total number of steps) of the method inaccordance with the invention can be executed by a computer (forexample, at least one computer). An embodiment of the computerimplemented method is a use of the computer for performing a dataprocessing method. An embodiment of the computer implemented method is amethod concerning the operation of the computer such that the computeris operated to perform one, more or all steps of the method.

The computer for example comprises at least one processor and forexample at least one memory in order to (technically) process the data,for example electronically and/or optically. The processor being forexample made of a substance or composition which is a semiconductor, forexample at least partly n- and/or p-doped semiconductor, for example atleast one of II-, III-, IV-, V-, VI-semiconductor material, for example(doped) silicon and/or gallium arsenide. The calculating steps describedare for example performed by a computer. Determining steps orcalculating steps are for example steps of determining data within theframework of the technical method, for example within the framework of aprogram. A computer is for example any kind of data processing device,for example electronic data processing device. A computer can be adevice which is generally thought of as such, for example desktop PCs,notebooks, netbooks, etc., but can also be any programmable apparatus,such as for example a mobile phone or an embedded processor. A computercan for example comprise a system (network) of “sub-computers”, whereineach sub-computer represents a computer in its own right. The term“computer” includes a cloud computer, for example a cloud server. Theterm “cloud computer” includes a cloud computer system which for examplecomprises a system of at least one cloud computer and for example aplurality of operatively interconnected cloud computers such as a serverfarm. Such a cloud computer is preferably connected to a wide areanetwork such as the world wide web (WWW) and located in a so-calledcloud of computers which are all connected to the world wide web. Suchan infrastructure is used for “cloud computing”, which describescomputation, software, data access and storage services which do notrequire the end user to know the physical location and/or configurationof the computer delivering a specific service. For example, the term“cloud” is used in this respect as a metaphor for the Internet (worldwide web). For example, the cloud provides computing infrastructure as aservice (IaaS). The cloud computer can function as a virtual host for anoperating system and/or data processing application which is used toexecute the method of the invention. The cloud computer is for examplean elastic compute cloud (EC2) as provided by Amazon Web Services™. Acomputer for example comprises interfaces in order to receive or outputdata and/or perform an analogue-to-digital conversion. The data are forexample data which represent physical properties and/or which aregenerated from technical signals. The technical signals are for examplegenerated by means of (technical) detection devices (such as for exampledevices for detecting marker devices) and/or (technical) analyticaldevices (such as for example devices for performing (medical) imagingmethods), wherein the technical signals are for example electrical oroptical signals. The technical signals for example represent the datareceived or outputted by the computer. The computer is preferablyoperatively coupled to a display device which allows informationoutputted by the computer to be displayed, for example to a user. Oneexample of a display device is an augmented reality device (alsoreferred to as augmented reality glasses) which can be used as “goggles”for navigating. A specific example of such augmented reality glasses isGoogle Glass (a trademark of Google, Inc.). An augmented reality devicecan be used both to input information into the computer by userinteraction and to display information outputted by the computer.Another example of a display device would be a standard computer monitorcomprising for example a liquid crystal display operatively coupled tothe computer for receiving display control data from the computer forgenerating signals used to display image information content on thedisplay device. A specific embodiment of such a computer monitor is adigital lightbox. The monitor may also be the monitor of a portable, forexample handheld, device such as a smart phone or personal digitalassistant or digital media player.

The expression “acquiring data” for example encompasses (within theframework of a computer implemented method) the scenario in which thedata are determined by the computer implemented method or program.Determining data for example encompasses measuring physical quantitiesand transforming the measured values into data, for example digitaldata, and/or computing the data by means of a computer and for examplewithin the framework of the method in accordance with the invention. Themeaning of “acquiring data” also for example encompasses the scenario inwhich the data are received or retrieved by the computer implementedmethod or program, for example from another program, a previous methodstep or a data storage medium, for example for further processing by thecomputer implemented method or program. Generation of the data to beacquired may but need not be part of the method in accordance with theinvention. The expression “acquiring data” can therefore also forexample mean waiting to receive data and/or receiving the data. Thereceived data can for example be inputted via an interface. Theexpression “acquiring data” can also mean that the computer implementedmethod or program performs steps in order to (actively) receive orretrieve the data from a data source, for instance a data storage medium(such as for example a ROM, RAM, database, hard drive, etc.), or via theinterface (for instance, from another computer or a network). The dataacquired by the disclosed method or device, respectively, may beacquired from a database located in a data storage device which isoperably to a computer for data transfer between the database and thecomputer, for example from the database to the computer. The computeracquires the data for use as an input for steps of determining data. Thedetermined data can be output again to the same or another database tobe stored for later use. The database or database used for implementingthe disclosed method can be located on network data storage device or anetwork server (for example, a cloud data storage device or a cloudserver) or a local data storage device (such as a mass storage deviceoperably connected to at least one computer executing the disclosedmethod). The data can be made “ready for use” by performing anadditional step before the acquiring step. In accordance with thisadditional step, the data are generated in order to be acquired. Thedata are for example detected or captured (for example by an analyticaldevice). Alternatively or additionally, the data are inputted inaccordance with the additional step, for instance via interfaces. Thedata generated can for example be inputted (for instance into thecomputer). In accordance with the additional step (which precedes theacquiring step), the data can also be provided by performing theadditional step of storing the data in a data storage medium (such asfor example a ROM, RAM, CD and/or hard drive), such that they are readyfor use within the framework of the method or program in accordance withthe invention. The step of “acquiring data” can therefore also involvecommanding a device to obtain and/or provide the data to be acquired. Inparticular, the acquiring step does not involve an invasive step whichwould represent a substantial physical interference with the body,requiring professional medical expertise to be carried out and entailinga substantial health risk even when carried out with the requiredprofessional care and expertise. In particular, the step of acquiringdata, for example determining data, does not involve a surgical step andin particular does not involve a step of treating a human or animal bodyusing surgery or therapy. In order to distinguish the different dataused by the present method, the data are denoted (i.e. referred to) as“XY data” and the like and are defined in terms of the information whichthey describe, which is then preferably referred to as “XY information”and the like.

The invention also relates to a program which, when running on acomputer, causes the computer to perform one or more or all of themethod steps described herein and/or to a program storage medium onwhich the program is stored (in particular in a non-transitory form)and/or to a computer comprising said program storage medium and/or to a(physical, for example electrical, for example technically generated)signal wave, for example a digital signal wave, carrying informationwhich represents the program, for example the aforementioned program,which for example comprises code means which are adapted to perform anyor all of the method steps described herein.

The invention also relates to a navigation system for computer-assistedsurgery, comprising:

the computer of the preceding claim, for processing the absolute pointdata and the relative point data;

a detection device for detecting the position of the main and auxiliarypoints in order to generate the absolute point data and to supply theabsolute point data to the computer;

a data interface for receiving the relative point data and for supplyingthe relative point data to the computer; and

a user interface for receiving data from the computer in order toprovide information to the user, wherein the received data are generatedby the computer on the basis of the results of the processing performedby the computer.

Within the framework of the invention, computer program elements can beembodied by hardware and/or software (this includes firmware, residentsoftware, micro-code, etc.). Within the framework of the invention,computer program elements can take the form of a computer programproduct which can be embodied by a computer-usable, for examplecomputer-readable data storage medium comprising computer-usable, forexample computer-readable program instructions, “code” or a “computerprogram” embodied in said data storage medium for use on or inconnection with the instruction-executing system. Such a system can be acomputer; a computer can be a data processing device comprising meansfor executing the computer program elements and/or the program inaccordance with the invention, for example a data processing devicecomprising a digital processor (central processing unit or CPU) whichexecutes the computer program elements, and optionally a volatile memory(for example a random access memory or RAM) for storing data used forand/or produced by executing the computer program elements. Within theframework of the present invention, a computer-usable, for examplecomputer-readable data storage medium can be any data storage mediumwhich can include, store, communicate, propagate or transport theprogram for use on or in connection with the instruction-executingsystem, apparatus or device. The computer-usable, for examplecomputer-readable data storage medium can for example be, but is notlimited to, an electronic, magnetic, optical, electromagnetic, infraredor semiconductor system, apparatus or device or a medium of propagationsuch as for example the Internet. The computer-usable orcomputer-readable data storage medium could even for example be paper oranother suitable medium onto which the program is printed, since theprogram could be electronically captured, for example by opticallyscanning the paper or other suitable medium, and then compiled,interpreted or otherwise processed in a suitable manner. The datastorage medium is preferably a non-volatile data storage medium. Thecomputer program product and any software and/or hardware described hereform the various means for performing the functions of the invention inthe example embodiments. The computer and/or data processing device canfor example include a guidance information device which includes meansfor outputting guidance information. The guidance information can beoutputted, for example to a user, visually by a visual indicating means(for example, a monitor and/or a lamp) and/or acoustically by anacoustic indicating means (for example, a loudspeaker and/or a digitalspeech output device) and/or tactilely by a tactile indicating means(for example, a vibrating element or a vibration element incorporatedinto an instrument). For the purpose of this document, a computer is atechnical computer which for example comprises technical, for exampletangible components, for example mechanical and/or electroniccomponents. Any device mentioned as such in this document is a technicaland for example tangible device.

A landmark is a defined element of an anatomical body part which isalways identical or recurs with a high degree of similarity in the sameanatomical body part of multiple patients. Typical landmarks are forexample the epicondyles of a femoral bone or the tips of the transverseprocesses and/or dorsal process of a vertebra. The points (main pointsor auxiliary points) can represent such landmarks. A landmark which lieson (for example on the surface of) a characteristic anatomical structureof the body part can also represent said structure. The landmark canrepresent the anatomical structure as a whole or only a point or part ofit. A landmark can also for example lie on the anatomical structure,which is for example a prominent structure. An example of such ananatomical structure is the posterior aspect of the iliac crest. Anotherexample of a landmark is one defined by the rim of the acetabulum, forinstance by the center of said rim. In another example, a landmarkrepresents the bottom or deepest point of an acetabulum, which isderived from a multitude of detection points. Thus, one landmark can forexample represent a multitude of detection points. As mentioned above, alandmark can represent an anatomical characteristic which is defined onthe basis of a characteristic structure of the body part. Additionally,a landmark can also represent an anatomical characteristic defined by arelative movement of two body parts, such as the rotational center ofthe femur when moved relative to the acetabulum.

The information on the imaging geometry preferably comprises informationwhich allows the analysis image (x-ray image) to be calculated, given aknown relative position between the imaging geometry analysis apparatusand the analysis object (anatomical body part) to be analyzed by x-rayradiation, if the analysis object which is to be analyzed is known,wherein “known” means that the spatial geometry (size and shape) of theanalysis object is known. This means for example that three-dimensional,“spatially resolved” information concerning the interaction between theanalysis object (anatomical body part) and the analysis radiation (x-rayradiation) is known, wherein “interaction” means for example that theanalysis radiation is blocked or partially or completely allowed to passby the analysis object. The location and in particular orientation ofthe imaging geometry is for example defined by the position of the x-raydevice, for example by the position of the x-ray source and the x-raydetector and/or for example by the position of the multiplicity(manifold) of x-ray beams which pass through the analysis object and aredetected by the x-ray detector. The imaging geometry for exampledescribes the position (i.e. the location and in particular theorientation) and the shape (for example, a conical shape exhibiting aspecific angle of inclination) of said multiplicity (manifold). Theposition can for example be represented by the position of an x-ray beamwhich passes through the center of said multiplicity or by the positionof a geometric object (such as a truncated cone) which represents themultiplicity (manifold) of x-ray beams. Information concerning theabove-mentioned interaction is preferably known in three dimensions, forexample from a three-dimensional CT, and describes the interaction in aspatially resolved way for points and/or regions of the analysis object,for example for all of the points and/or regions of the analysis object.Knowledge of the imaging geometry for example allows the location of asource of the radiation (for example, an x-ray source) to be calculatedrelative to an image plane (for example, the plane of an x-raydetector). With respect to the connection between three-dimensionalanalysis objects and two-dimensional analysis images as defined by theimaging geometry, reference is made for example to the followingpublications:

-   1. “An Efficient and Accurate Camera Calibration Technique for 3D    Machine Vision”, Roger Y. Tsai, Proceedings of the IEEE Conference    on Computer Vision and Pattern Recognition. Miami Beach, Fla., 1986,    pages 364-374-   2. “A Versatile Camera Calibration Technique for High-Accuracy 3D    Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses”,    Roger Y. Tsai, IEEE Journal of Robotics and Automation, Volume RA-3,    No. 4, August 1987, pages 323-344.-   3. “Fluoroscopic X-ray Image Processing and Registration for    Computer-Aided Orthopedic Surgery”, Ziv Yaniv-   4. EP 08 156 293.6-   5. U.S. 61/054,187

Preferably, atlas data is acquired which describes (for example defines,more particularly represents and/or is) a general three-dimensionalshape of the anatomical body part. The atlas data therefore representsan atlas of the anatomical body part. An atlas typically consists of aplurality of generic models of objects, wherein the generic models ofthe objects together form a complex structure. For example, the atlasconstitutes a statistical model of a patient's body (for example, a partof the body) which has been generated from anatomic information gatheredfrom a plurality of human bodies, for example from medical image datacontaining images of such human bodies. In principle, the atlas datatherefore represents the result of a statistical analysis of suchmedical image data for a plurality of human bodies. This result can beoutput as an image—the atlas data therefore contains or is comparable tomedical image data. Such a comparison can be carried out for example byapplying an image fusion algorithm which conducts an image fusionbetween the atlas data and the medical image data. The result of thecomparison can be a measure of similarity between the atlas data and themedical image data. The atlas data comprises positional informationwhich can be matched (for example by applying an elastic or rigid imagefusion algorithm) for example to positional information contained inmedical image data so as to for example compare the atlas data to themedical image data in order to determine the position of anatomicalstructures in the medical image data which correspond to anatomicalstructures defined by the atlas data.

The human bodies, the anatomy of which serves as an input for generatingthe atlas data, advantageously share a common feature such as at leastone of gender, age, ethnicity, body measurements (e.g. size and/or mass)and pathologic state. The anatomic information describes for example theanatomy of the human bodies and is extracted for example from medicalimage information about the human bodies. The atlas of a femur, forexample, can comprise the head, the neck, the body, the greatertrochanter, the lesser trochanter and the lower extremity as objectswhich together make up the complete structure. The atlas of a brain, forexample, can comprise the telencephalon, the cerebellum, thediencephalon, the pons, the mesencephalon and the medulla as the objectswhich together make up the complex structure. One application of such anatlas is in the segmentation of medical images, in which the atlas ismatched to medical image data, and the image data are compared with thematched atlas in order to assign a point (a pixel or voxel) of the imagedata to an object of the matched atlas, thereby segmenting the imagedata into objects.

In particular, the invention does not involve or in particular compriseor encompass an invasive step which would represent a substantialphysical interference with the body requiring professional medicalexpertise to be carried out and entailing a substantial health risk evenwhen carried out with the required professional care and expertise. Forexample, the invention does not comprise a step of positioning a medicalimplant in order to fasten it to an anatomical structure or a step offastening the medical implant to the anatomical structure or a step ofpreparing the anatomical structure for having the medical implantfastened to it. More particularly, the invention does not involve or inparticular comprise or encompass any surgical or therapeutic activity.The invention is instead directed as applicable to positioning a toolrelative to the medical implant, which may be outside the patient'sbody. For this reason alone, no surgical or therapeutic activity and inparticular no surgical or therapeutic step is necessitated or implied bycarrying out the invention.

The present invention can be implemented, for example in navigationsoftware for hip surgery. The software may therefore import an X-raydataset and, on that basis, calculate the above described sagittalrotation between the X-ray image plane and an AAC-plane, which willeventually allow a navigation on a functional plane defined by thepreoperative X-ray dataset.

BRIEF DESCRIPTION OF DRAWINGS

In the following, the invention is described with reference to theenclosed figures which represent preferred embodiments of the invention.The scope of the invention is not however limited to the specificfeatures disclosed in the figures, which show:

FIG. 1 a geometric setup of X-ray-imaging;

FIG. 2 an anterior-posterior X-ray-image with calibration features;

FIG. 3 shows the image of FIG. 2 with pelvic landmarks indicated;

FIG. 4 a sagittal rotation angle between an X-ray plane and an AACplane;

FIG. 5 a registration of an AAC-plane;

FIG. 6 the basic steps of the inventive computer implemented method.

DETAILED DESCRIPTION

The geometric setup of a conventional X-ray-imaging method can be seenin FIG. 1. A punctual radiation source 12 emits radiation in a sphericalmanner, and towards a pelvis (not shown). As the emitted radiation isadsorbed by matter to a varying degree, an image 5 of the pelvis 1 formsbehind the pelvis 1 in an image plane (not shown). This image is howeverdistorted due to the spherical propagation of the emitted radiation. Anexemplary landmark 3 will therefore be depicted at a position in theimage which is different from a position that would have been obtainedif the entire pelvis 1 is projected into the image plane in ananterior-posterior direction 2.

According to one embodiment the present invention, this positional shiftor distortion is compensated for by determining the geometric setupincluding the distance between the X-ray-source 12 and the calibrationplane 4, and the distances 13 of each of the pelvic landmarks 3 to thecalibration plane 4 in the anterior-posterior direction 2.

After an X-ray-image of the patient's pelvis together with a pluralityof calibration features 9, 10 has been taken in an anterior-posteriordirection 2 (shown in FIG. 1), a plurality of pelvic landmarks 3, 14, 15can then be identified either manually or automatically within theX-ray-image as shown in FIG. 3.

After the X-ray-image has been “calibrated” by performing steps 2, 3, 4and 5 of the inventive method outlined in FIG. 6, distance measurementsbetween the pelvic landmarks 3, 14, 15 in a media-lateral direction andin a cranial-caudal direction are possible. The determined distances 6,7 can then be compared with statistical values obtained from a pluralityof reference patients.

Since a database stored on a storage medium provides a directcorrelation between the distance measurements and a determined value forthe sagittal rotation 8 of the pelvis 1, the sagittal rotation 8 of thecurrent patient's pelvis can be derived from that database.

According to a preferred embodiment of the present invention, thesagittal rotation 8 defines the angle between the coronal plane 4 and anAAC-plane 16 (shown in FIG. 4). The AAC-plane 16 can serve as areference plane for procedures following the inventive method, after theactual pelvis 1 has been registered in real space, for example bydetermining the spatial position of the left and the rightanterior-superior iliac spine and one of the centers of rotation of theleft or right acetabulum, as shown in FIG. 5.

The invention claimed is:
 1. A computer implemented method fordetermining a sagittal rotation of a patient's pelvis, the methodcomprising: acquiring image data describing a two-dimensionalX-ray-image of the patient's pelvis made in an anterior-posteriordirection; determining, based on the image data, position datadescribing the position of a plurality of pelvic landmarks reproduced inor derivable from the two-dimensional X-ray-image; acquiring calibrationdata describing the position of a calibration plane with respect to thepatient's pelvis, the calibration plane being perpendicular to theanterior-posterior direction; determining, based on the position dataand the calibration data, calibrated projection data describing atwo-dimensional projection representing the plurality of pelviclandmarks of the patient's pelvis being projected into the calibrationplane in the anterior-posterior direction; acquiring distance datadescribing at least one of a medial-lateral distance and acranial-caudal distance between at least one first pelvic landmark andat least one second pelvic landmark in the calibrated two-dimensionalprojection; acquiring, from a database, regression data describing alinear correspondence between at least one of said medial-lateraldistances and cranial-caudal distances, and a pelvic sagittal rotation;and determining, based on the distance data and the regression data, thesagittal rotation of the patient's pelvis.
 2. The method of claim 1,wherein the determining calibrated projection data involves the use ofat least one calibration feature of a predetermined size and/orgeometry, which is reproduced in the X-ray-image.
 3. The method of claim2, wherein at least one calibration feature is disposed anterior to thepatient's pelvis and at least one calibration feature is disposedposterior to the patient's pelvis.
 4. The method of claim 1, wherein thedetermining calibrated projection data comprises: acquiringsource-distance data describing the distance between the X-ray-sourceand the calibration plane; and acquiring anterior-posterior-distancedata describing the distance of each of the plurality of pelviclandmarks to the calibration plane in the anterior-posterior direction.5. The method of claim 4, wherein: predefined values are set for theanterior-posterior-distances of the plurality of pelvic landmarks, orwherein the acquiring anterior-posterior-distance data comprises:matching a three-dimensional pelvis model acquired from an anatomicalatlas to the two-dimensional X-ray-image, the model specifying theposition of the plurality of pelvic landmarks; and identifying, based onthe three-dimensional pelvis model, statistical values for theanterior-posterior-distances of the plurality of pelvic landmarks to thecalibration plane in the anterior-posterior direction.
 6. The method ofclaim 1, wherein the determining calibrated projection data comprises:acquiring reference distance data describing the distance in theanterior-posterior direction of each of the plurality of pelviclandmarks to at least one of: an anterior reference plane defined by theat least one calibration feature disposed anterior to the patient'spelvis; and a posterior reference plane defined by the at least onecalibration feature disposed posterior to the patient's pelvis;acquiring plane distance data describing the distance in theanterior-posterior direction of the calibration plane to at least one ofthe anterior reference plane and the posterior reference plane; anddetermining, based on the reference distance data and the plane distancedata, anterior-posterior-distance data describing the distance of eachof the plurality of pelvic landmarks to the calibration plane in theanterior-posterior direction.
 7. The method of claim 1, wherein thedetermining position data comprises: matching a three-dimensional pelvismodel acquired from an anatomical atlas to the two-dimensionalX-ray-image, the three-dimensional pelvis model specifying the positionof the plurality of pelvic landmarks; and identifying, based on thethree-dimensional pelvis model, the position of the plurality of pelviclandmarks within the two-dimensional X-ray-image.
 8. The method of claim1, wherein the at least one first pelvic landmark and the at least onesecond pelvic landmark are selected from the group consisting of: leftanterior superior iliac spine, right anterior superior iliac spine; leftiliosacral joint, right left iliosacral; left lateral foramen point,right lateral foramen point; cranial edge of pubic symphysis; and centerof rotation of left acetabulum, center of rotation of right acetabulum.9. The method of claim 1, wherein the at least one of the medial-lateraldistance and the cranial-caudal distance between the at least one firstpelvic landmark and the at least one second pelvic landmark are selectedfrom the group consisting of: medial-lateral distance between leftanterior superior iliac spine and right anterior superior iliac spine;medial-lateral distance between iliosacral joint and line connectinganterior superior iliac spines; cranial-caudal distance between centerof rotation of left or right acetabulum and line connecting anteriorsuperior iliac spines; cranial-caudal distance between center ofrotation of left or right acetabulum and intersection point ofmidsagittal plane and line connecting left lateral foramen point andright lateral foramen point; medial-lateral distance between center ofrotation of left or right acetabulum and midsagittal plane of patient;and cranial-caudal distance between center of rotation of left or rightacetabulum and cranial edge of pubic symphysis.
 10. The method of claim1, wherein the two-dimensional X-ray-image shows the patient's pelvis ofthe patient taking a standing posture.
 11. The method of claim 1,wherein the regression data is based on measurements acquired from aplurality of reference patients, wherein for each one of the referencepatients, a correspondence between a pelvic sagittal rotation and atleast one of a medial-lateral distance and a cranial-caudal distancebetween the at least one first pelvic landmark and the at least onesecond pelvic landmark within the calibration plane is described. 12.The method of claim 1, wherein the sagittal rotation of the patient'spelvis defines the angle between the coronal plane of the patient and anAAC-plane containing the left anterior superior iliac spine, the rightanterior superior iliac spine, and at least one of the center ofrotation of the left acetabulum and the center of rotation of the rightacetabulum.
 13. The method of claim 12, wherein the determined AAC-planeis passed on, for further use as a reference basis, to a registrationprocedure for surgery.
 14. A system for determining a sagittal rotationof a patient's pelvis, comprising: a computer configured to: acquireimage data describing a two-dimensional X-ray-image of the patient'spelvis made in an anterior-posterior direction; determine, based on theimage data, position data describing the position of a plurality ofpelvic landmarks reproduced in or derivable from the two-dimensionalX-ray-image; acquire calibration data describing the position of acalibration plane with respect to the patient's pelvis, the calibrationplane being perpendicular to the anterior-posterior direction;determine, based on the position data and the calibration data,calibrated projection data describing a two-dimensional projectionrepresenting the plurality of pelvic landmarks of the actual patient'spelvis being projected into the calibration plane in theanterior-posterior direction; acquire distance data describing at leastone of a medial-lateral distance and a cranial-caudal distance betweenat least one first pelvic landmark and at least one second pelviclandmark in the calibrated two-dimensional projection; acquire, from adatabase, regression data describing a linear correspondence between atleast one of said medial-lateral distances and cranial-caudal distances,and a pelvic sagittal rotation; and determine, based on the distancedata and the regression data, the sagittal rotation of the patient'spelvis.
 15. The system of claim 14, wherein the determining calibratedprojection data involves the use of at least one calibration feature ofa predetermined size and/or geometry, which is reproduced in theX-ray-image.
 16. The system of claim 15, wherein at least onecalibration feature is disposed anterior to the patient's pelvis and atleast one calibration feature is disposed posterior to the patient'spelvis.
 17. The system of claim 14, wherein the determining calibratedprojection data comprises: acquiring source-distance data describing thedistance between the X-ray-source and the calibration plane; andacquiring anterior-posterior-distance data describing the distance ofeach of the plurality of pelvic landmarks to the calibration plane inthe anterior-posterior direction.
 18. The system of claim 14, whereinthe determining calibrated projection data comprises: acquiringreference distance data describing the distance in theanterior-posterior direction of each of the plurality of pelviclandmarks to at least one of: an anterior reference plane defined by theat least one calibration feature disposed anterior to the patient'spelvis; and a posterior reference plane defined by the at least onecalibration feature disposed posterior to the patient's pelvis;acquiring plane distance data describing the distance in theanterior-posterior direction of the calibration plane to at least one ofthe anterior reference plane and the posterior reference plane;determining, based on the reference distance data and the plane distancedata, anterior-posterior-distance data describing the distance of eachof the plurality of pelvic landmarks to the calibration plane in theanterior-posterior direction.
 19. The system of claim 14, wherein thedetermining position data comprises: matching a three-dimensional pelvismodel acquired from an anatomical atlas to the two-dimensionalX-ray-image, the three-dimensional pelvis model specifying the positionof the plurality of pelvic landmarks; and identifying, based on thethree-dimensional pelvis model, the position of the plurality of pelviclandmarks within the two-dimensional X-ray-image.
 20. A non-transitorycomputer readable storage medium storing a computer program fordetermining a sagittal rotation of a patient's pelvis which, whenrunning on a computer or loaded onto the computer, causes the computerto: acquire image data describing a two-dimensional X-ray-image of thepatient's pelvis made in an anterior-posterior direction; determine,based on the image data, position data describing the position of aplurality of pelvic landmarks reproduced in or derivable from thetwo-dimensional X-ray-image; acquire calibration data describing theposition of a calibration plane with respect to the patient's pelvis,the calibration plane being perpendicular to the anterior-posteriordirection; determine, based on the position data and the calibrationdata, calibrated projection data describing a two-dimensional projectionrepresenting the plurality of pelvic landmarks of the patient's pelvisbeing projected into the calibration plane in the anterior-posteriordirection; acquire distance data describing at least one of amedial-lateral distance and a cranial-caudal distance between at leastone first pelvic landmark and at least one second pelvic landmark in thecalibrated two-dimensional projection; acquire, from a database,regression data describing a linear correspondence between at least oneof said medial-lateral distances and cranial-caudal distances, and apelvic sagittal rotation; and determine, based on the distance data andthe regression data, the sagittal rotation of the patient's pelvis.